Cells derived from patients with xeroderma pigmentosum, who are predisposed to sunlight induced skin cancer, are unable to catalyze incision of DNA damaged by light as well as by a variety of structurally unrelated carcinogens. The three structural genes responsible for this enzymes reaction in E. coli have been cloned and their gene product uvrA, uvrB and uvrC proteins have been isolated determinations and for their use as antigens. These three genes are to be separately introduced into the heterologous composite pBR322-SV40 plasmid, pSV2-gpt, containing the E. coli gene (gpt) which encodes the xanthine guanine phosphoribosyl transferase (XGPRT) gene (Mulligan, R.C. and Berg, P., (1980) Science 209, 1427). Cotransfection of the uvr genes with gpt into monkey kidney cells (BSC-1) and human skin fibroblasts in a medium selectively supporting growth of only those cells expressing gpt greatly facilitates selection for those cells cotransfected with uvr genes. Transfected cells will be screened for XGPRTase, HPRTase, gpt mRNA, uvr mRNA uvr DNA fluorescene. The reversion of those repair deficient cells expressing uvr genes to a state of repair proficiency will be determined.